This invention relates to a coherent optical communication system in which a laser is used as an optical source.
In a transmitter of the optical communication system, a laser is used as a transmitter optical source for generating an optical carrier signal of a carrier frequency. The carrier signal is modulated into a modulated optical signal by an electrical modulating signal which usually varies as a binary signal between binary one and zero levels or mark and space levels at a modulating frequency, namely, at a bit rate, selected typically between ten kilobits and ten gigabits per second in compliance with the number of information channels carried by the modulating signal, In such an optical communication system, optical heterodyne or homodyne detection provides a high reception sensitivity which is ten to one hundred times as high as that achieved by direct optical detection. On carrying out the optical heterodyne or homodyne detection, the optical carrier signal is modulated in accordance with amplitude shift keying (ASK), frequency shift keying (FSK), or phase shift keying (PSK).
As will readily be understood, a receiver of the optical communication system comprises a local optical source for generating a local optical signal of a local oscillation frequency predetermined in compliance with one of the optical heterodyne and the optical homodyne detection techniques that should be carried out. An optical mixer is for mixing the local optical signal and the modulated optical signal which is received through an optical path from the transmitter. The mixer thereby produces a mixed optical signal. A demodulator demodulates the mixed optical signal into a demodulated electrical signal which represents the modulating signal.
When the amplitude shift keying is resorted to, the requisites are not strict as regards the spectral width and the frequency stability of the optical carrier signal. The transmitter may therefore be of a simple structure. The modulated optical signal, however, has a reduced output power when the optical heterodyne or homodyne detection is additionally used, with a semiconductor laser used as the transmitter optical source. This is because the semiconductor laser must be modulated by the use of an external modulator which inevitably gives rise to an insertion loss to reduce the output power. It may be mentioned here that a semiconductor laser can directly be intensity modulated by varying the injection current thereof in accordance with the modulating signal. In this event, the carrier frequency unavoidably varies with the injection current. This is the reason why the external modulator is necessary under the circumstances.
For the frequency shift keying, it is possible to directly frequency modulate a semiconductor laser by varying the injection current in compliance with the modulating signal. Furthermore, the reception sensitivity is theoretically raised 3 dB as compared with that achieved by the optical heterodyne or homodyne reception with the amplitude shift keying. The requisites are, however, strict for the spectral width and the frequency stability. The present-day semiconductor lasers are not all satisfactory for these requisites. When the requisites are not satisfied, the reception sensitivity is adversely affected. The injection current varied with the modulating signal, will be called a modulating current merely for convenience of the following description.
It is already reported as will later be described more in detail that a frequency shift per unit modulating current is dependent on the modulation frequency when the direct frequency modulation is resorted to. As a consequence, the modulated optical signal is not only frequency modulated but also intensity modulated. This eventually results in a distortion of the demodulated electrical signal.